The present invention relates to a driver circuit for lamps, particularly but not exclusively arc discharge lamps, which conventionally require a higher voltage to commence operation than is required to sustain a discharge.
Arc discharge lamps require an alternating current source of a relatively low frequency to drive the lamp. In addition, to start the lamp, a very high voltage pulse is usually required. Such lamps are well known, common examples of lamps using metal halide technology; the details of the lamp itself are not critical to the present invention.
A conventional circuit for driving an arc discharge lamp is illustrated schematically in FIG. 1. This circuit uses a high frequency switching transistor Q1 connected to an inductor I1 to produce a controlled current of a suitable value for driving the lamp. The high voltage is then converted to an alternating voltage of relatively low frequency by a switching bridge network comprising transistors Q2-5. The transistors are driven via respective isolators DR1-5 by integrated controller 31. Ignition is provided by a pulse transformer 15 in series with the lamp 13, which, on actuation, generates a very high voltage pulse of the order of 15 KV to 40 KV to start the lamp.
A problem with the conventional circuit is that the use of such a high starting voltage can present problems. In particular, the circuitry needs to be heavily insulated to withstand voltages much higher than those that are present during normal operation of the lamp. This leads to increased cost, and often increased size of the apparatus. In addition, the high voltages present a severe shock hazard.
If the frequency of the alternating current is increased to a higher frequency, typically an ultrasonic frequency, a capacitive displacement current flows between the electrodes of the lamp, which provides ionisation and facilitates ignition at lower strike voltages. However, high frequencies are undesirable for the operation of the lamp, due to the tendency to produce acoustic standing waves in the lamp that can disturb the arc path, and cause lamp flicker and arc instability.
U.S. Pat. No. 5,491,386 discloses a circuit for powering a high-pressure discharge lamp in which a high frequency, greater than the resonant frequency of a tuned circuit is employed to start the lamp, and a lower frequency is used thereafter. A problem with this arrangement, which relies entirely on the tuned circuit to generate an increased voltage for starting the lamp is that the circuit may be unstable around the time of starting the lamp; a complex protocol for controlling the switching frequency is required to alleviate this.
EP-A-408,121 discloses a lamp driver circuit in which a pair of auto-transformers on a common core are employed in conjunction with a capacitor to provide increased voltage to start the lamp. This arrangement requires a purpose built auto-transformer, in which both the inductance and turns ratio of each of four coils must be controlled in order to achieve the desired resonance. Additionally, a sensing coil, and complex control circuitry are required.